Orbital degree of freedom induced non-Hermitian skin effect in acoustic crystals

Topological phases of matter have been extensively studied in artificial crystals and metamaterials. Recent advances in non-Hermitian topology have revealed unique phenomena such as the non-Hermitian skin effect (NHSE). While spin and valley degrees of freedom (DoFs) have been widely explored, orbital DoFs, which can generate distinct topological effects, remain understudied in non-Hermitian systems, particularly in the context of point-gap topology. Here, we theoretically and numerically investigate the orbital NHSE in a one-dimensional zigzag Su-Schrieffer-Heeger acoustic crystal. We demonstrate the NHSE for both ${p}_{x}$ and ${p}_{y}$ orbital modes through unidirectional nearest-neighbor couplings and show that the NHSE direction can be reversed by tuning the bonding angle. Furthermore, by introducing third-nearest-neighbor unidirectional couplings, we achieve an orbital bipolar NHSE, where ${p}_{x}$ and ${p}_{y}$ modes localize at opposite boundaries within the same frequency range. These findings, supported by theoretical calculations and numerical simulations, highlight the unique role of orbital DoFs in non-Hermitian systems and provide new mechanisms for controlling wave localization. Our work paves the way for exploring orbital-induced phenomena in higher-dimensional and multiorbital systems.